The present invention relates to a field of surveillance systems and in particular to those surveillance systems that utilize a radar system to provide the monitoring of moving objects within a predetermined space that is to be guarded. The prior art includes television surveillance systems such as that of the Kartchner U.S. Pat. No. 3,531,588 which includes an automatic televison apparatus for continuously scanning a security-protected scene for periodically storing a complete frame of live television signals representing the scanned scene. The live television signals and the stored television signals are amplitude-compared and pulses representing any differences are coupled by digitalized logic circuits to alarm devices. Upon an alarm, the pulses actuate a gated limiter through which the television signals are applied to a picture tube whereby the signals are clamped to a white level upon each actuation thus outlining any intrusion within the scanned scene with a white border. Another type of surveillance system is that of the Schwartz, et al., U.S. Pat. No. 3,314,066 in which a CW microwave transmitter is placed at one extremity of the scene to be guarded for sending energy to a reflector at the other extremity of the guarded scene. In this system, the intensity of the transmitted signal is modulated by hostile objects that enter the radar beam between the transmitting antenna and the reflector. The modulation energy of the reflected radar beam is analyzed for target size to produce an alarm signal when a hostile object is detected.
Still another type of surveillance system is that of the J. D. Dautel et al. patent application Ser. No. 202,997, now U.S. Pat. No. 3,815,131, assigned to the same assignee as is the present invention, in which two CW microwave transmitter antennas are positioned facing one another within a space that is to be guarded. Each transmitting antenna is located within the radar beam of the other facing transmitting antenna and operates at the same or a different CW frequency. A moving object within the surveillance space produces a doppler frequency shift in the echo return to each transmitting antenna which produces a detected target signal. AND/OR logic gates responsive to the detected target signals from each radar system provide an alarm or alert condition. The radar system has resultant range vs. object size characteristic over the surveillance space so that a small moving object, such as a small animal, will produce a detected signal in only one of the radar systems thereby indicating an alert condition while a large moving object, such as a person, will produce a detected signal in each of the radar systems thereby indicating an alarm condition. Radar signal returns from a large target, such as a moving vehicle, beyond the desired surveillance range will produce a detected signal in only one of the radars and will produce an alert signal but not an alarm signal.